Just as we saw with Gimmel and Dalet and then with Heh and Vav, the letters Zayin and Chet are also complementary. Zayin represents Creation, the physical struggles connected to it as well as the spirituality within it, such as the Shabat. Zayin means weapon, and Zan means to provide (materially). There is a deep connection between making a living and going to battle. The word for Lechem (bread/sustenance) is also found in the word Milchama (war).
Chet, on the other hand, represents tremendous power and inner qualities that are above nature. Chet stands for Cheit (sin), but also the ability to overcome sin and live (Chaim). (Munk) Rebbe Nachman of Breslov, in the opening pages of Likutei Moharan, explains that Cheit is Chiut (vitality), which is also the inner wisdom (Chochmah) and thinking (Sechel).
The Water and the Seas have a similar kind of relationship. Water is the source of life in the physical world. We ourselves are made of mostly water. Yet water also represents the Torah, and is deeply connected to all things spiritual. The Waters sing: "The sound of His voice places the multitude of waters in the heavens and He raises the vapors from the end of the earth." (Jeremiah 51:16) Water is connected to both heaven and earth; the material as well as the spiritual.
The Seas represent tremendous awe-inspiring might, deep below the surface, yet reflecting also G-d's tremendous heights. The Seas say: "More than the voices of many waters, mightier than the breakers of the sea, Hashem is mighty on high." (Psalms 93:4) The Seas also represent inner wisdom, waiting to be revealed, like in the splitting of the Sea of Reeds in our redemption from Egypt. (See Rabbi Slifkin's, "Nature's Song")